In long distance optical communication systems it may be important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the optical transmission cable, faulty repeaters or amplifiers, or other problems with the system. Known monitoring methods include use of optical time domain reflectometry (OTDR) equipment and techniques.
In more detail, and according to conventional OTDR techniques, an OTDR signal source generates a test or probe signal, such as an optical pulse or a specially modulated optical carrier, and the test signal is launched into the outbound optical path of a path pair. Elements in the outbound path may reflect (e.g., backscatter) portions of the OTDR test signal. The backscattered signal portions may be returned (e.g., on the same outbound path or a different path such as the inbound path) and detected in an OTDR receiver. The transmission characteristics of each element in the path may also affect the amount of signal reflected at points after that element, for example, by attenuating the test signal or the reflected signal. The magnitude of the backscattered or reflected signal from each element or point along the optical path may be used as a metric for characterizing the optical path. Coherent optical time domain reflectometry (COTDR) is an enhancement of OTDR and may be used in long-haul WDM systems such as undersea optical communication systems. COTDR uses a special optical modulation scheme for its test signal and a coherent optical detection receiver to improve receiver sensitivity. The improved sensitivity enables measurement of very low levels of backscattered signal and thus the examination of very long optical fibers even if the fibers are in portions of the optical path far from the COTDR equipment (e.g., beyond an optical amplifier). Because Rayleigh backscatter from optical fiber in the transmission path can be detected by OTDR or COTDR, this approach to system monitoring provides a diagnostic tool that allows the user to examine the fiber between repeaters.
The cost of an undersea optical cable system and other such long haul communication systems is significantly influenced by the number of repeaters. Thus, there is a continuing desire to expand the spacing between repeaters, so as to reduce the number of repeaters. Recently, the maximum possible repeater span has increased from about 50 km to more than 150 km with the introduction of advanced modulation formats like differential phase-shift keying (DPSK) and the increased pump power and two-stage amplification. However measurement capability of OTDR equipment has not improved in step, and is generally still limited to within 90 km. Therefore, about half of the repeater span may be immeasurable. Moreover, high loss loop back (HLLB) paths inside currently available repeaters only allow measuring reflected Rayleigh signals from the outgoing direction, because conventional architectures generally only have one path connecting from one amplifier output of a repeater to the other amplifier output of that repeater. As such, conventional architectures may not be able to measure the Rayleigh signal from the incoming fiber.